1. Field of the Invention
The present invention relates to record medium and apparatus useful in high density storage of information by optical writing and/or reading, and more particularly to improved configurations for optical disk write/read apparatus.
2. Description of the Prior Art
The currently preferred optical disk technology employs disk elements with spiral or concentric tracks of minute (e.g., on the order of a micron or less in size), optically-detectable marks. One real-time mode of recording (writing) such marks is by scanning tracks on the disk with an equivalently small beam of radiation (e.g., from a laser) which is modulated "off or on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with the same size or only slightly larger, but still very tightly focused, radiation (e.g. light) spot. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the read-out light reflected from the recorded disk.
In order to write and read information in the form of such minute markings, optical systems of high numerical aperture are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus and the proper positional relation between the writing or reading optical system, and the optical disk record surface must be stringently maintained both smooth and flat.
One approach to achieve requisite smoothness and flatness has been to form the disk substrate of glass with a ground and polished surface. This requires a time-consuming and costly fabrication procedure. Another approach is to mold a plastic disk substrate with a highly finished surface and apply a surface smoothing sub-layer. However, it is extremely difficult to mold such plastic disks having adequate surface characteristics with a good yield; and this support fabrication method is also relatively costly.
Still another approach for meeting smoothness, flatness and protective requirements is disclosed in commonly-assigned, copending U.S. patent application Ser. No. 264,313 filed May 18, 1981 in the names of F. F. Geyer and E. M. Leonard. In that approach an improved optical disk assembly adapted for high density storage of information comprises (i) a flexible, disk-shaped support carrying a record layer; (ii) a transparent disk cover sheet opposing the record layer, and (iii) an annular retaining ring for holding the support and cover sheet in a relatively low circumferentially-symmetric tension and tensioned to an operating tension when drawn over a rotatable annular locating surface on the disk assembly receiving structure of the optical disk write/read apparatus. Thus the disk assembly can normally be kept with the support material in low "storage" tension, and used with the support material in higher "operating" tension to enhance flatness.